I noticed that for our desktop devices the owner changes frequently. Means it changes character-case from lower to upper and vice versa on windows desktops. Normally we change the Registered Company field to the sub company where the device is delivered to and the Registered Owner to something like this: DOMAIN/Name (the users domain logon name).

On our regular rescans I get info about changed owner: DOMAIN/name. Some days later it changes back to: DOMAIN/Name. Its just a change in the Name from lower- to upper-case and back. But on 750+ devices it spams the timeline a lot.

Ist it a Windows/WMI problem? Or something else... is there something I can do? (Beside changing all owner to lower-case like domain/name)

Hmmm... makes sense, if the Device Owner ist not only made of the registry key RegisteredOwner, but also made of the last user logged in / or still using the PC.

Users normally use lower-case login name, and domain is always upper-case. And in RegisteredOwner we type the first letter in upper-case. Also have found a device where the name changes completely: RegisteredOwner is not the one logged in...

Is there a way to tell SW it should only use RegisteredOwner for owner in the device?

You can still search for the login name in SW. It will show you the machines where the user is listed in last logon. Another way is to use browse view (select group, select "Last Login", select the login name). Reports are nice too.